In district cooling or heating and central cooling or heating, the most important technique is to transfer heat energy from a heat generation place to widely scattered places of heat consumption. In the prior art, this transfer of heat energy has been carried out by using high-temperature water or steam as the medium. In either case, however, the dissipation loss of heat during its transport is substantial and, in an extreme case, as much as 50% of the effective heat energy is lost during its transport. This involves a serious problem in that the energy-saving nature of central cooling or heating is markedly impaired.
Moreover, since the heat-transfer medium such as high-temperature water or steam transported according to the prior art methods is directly utilized on the consumer side, heat storage functioning to maintain an adequate balance between heat generation and heat utilization cannot be achieved efficiently. Furthermore, especially where high-temperature water or low-pressure steam is used as the heat-transfer medium, a decrease in temperature of the heat-transfer medium provided for heat utilization as compared with its temperature at the heat generation place, i.e., the so-called degradation in the quality of energy, has been unavoidable.